The Swiftest Planet Sun-scorched Mercury is only slightly larger than Earth's moon. Like the moon, Mercury has very little atmosphere to stop impacts, and it is covered with craters. Mercury's dayside is super-heated by the sun, but at night temperatures drop hundreds of degrees below freezing. Ice may even exist in craters. Mercury's egg-shaped orbit takes it around the sun every 88 days.

The measure or property of a material to be resistant to flow; the internal friction of a material. Materials with a high viscosity are more resistant to flow, while those with a low viscosity are more fluid.

viscous relaxation

The process whereby topographic features become subdued over time due to the flow of the surrounding geologic material.

volatile

As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes hydrogen, helium, water, ammonia, carbon dioxide and methane.

volcanic

Produced or discharged from a volcano.

volcanism

The eruption of molten material or gas-driven solid fragments onto the surface of a planetary body. On icy moons the phenomenon is sometimes called cryovolcanism to distinguish it from silicate or sulfur volcanism. (Image: A volcano blasts material off the surface of Jupiter's moon Io.)

volcano

A mountain formed from the eruption of igneous matter through a source vent.

The measure or property of a material to be resistant to flow; the internal friction of a material. Materials with a high viscosity are more resistant to flow, while those with a low viscosity are more fluid.

viscous relaxation

The process whereby topographic features become subdued over time due to the flow of the surrounding geologic material.

volatile

As a noun, this refers to substances that are gases at ordinary temperatures. In astronomy it includes hydrogen, helium, water, ammonia, carbon dioxide and methane.

volcanic

Produced or discharged from a volcano.

volcanism

The eruption of molten material or gas-driven solid fragments onto the surface of a planetary body. On icy moons the phenomenon is sometimes called cryovolcanism to distinguish it from silicate or sulfur volcanism. (Image: A volcano blasts material off the surface of Jupiter's moon Io.)

volcano

A mountain formed from the eruption of igneous matter through a source vent.